''There has been discussion about whether children with autism have high levels [of mercury],'' says the study's lead author, Irva Hertz-Picciotto, PhD, an epidemiologist, professor, and chief of environmental and occupational health at the University of California, Davis, and a researcher at the MIND Institute there.
Hertz-Picciotto cautioned that her recent study does not examine whether mercury plays a role in causing the disorder, which has been the focus of ongoing debate. Major studies of children who were given vaccines with the mercury-containing preservative thimerosal (now phased out of most vaccines given to children) don't find a link between the vaccines and autism, but some organizations led by parents of autistic children doubt those conclusions.
The blood levels in the study were taken after a child had already received a diagnosis of autism, a developmental disorder now believed to affect one in 91 U.S. children and marked by difficulty in communication, social interaction, and learning.
Some took exception with the new study.
''Measuring blood levels of mercury is a useless way to assess chronic damage or pathology from mercury, as it clears the bloodstream relatively rapidly," says Jim Moody, a director for the Coalition for SafeMinds (Sensible Action for Ending Mercury-Induced Neurological Disorders), an organization that investigates the risks of mercury exposure.
Mercury Levels and Autism: Study Details
Triggering the research, Hertz-Picciotto says, is that some researchers have thought children with autism may have higher levels of mercury in their blood because their bodies don't get rid of it it as efficiently as other children and that buildup might be contributing to the problems.
But others have speculated that children with autism may have lower blood levels of mercury because the mercury is sequestered in their brain, she says.
For the study, Hertz-Picciotto and her colleagues compared the blood levels of mercury in 249 children with autism or autism spectrum disorder, in 143 typically developing children and in 60 children with developmental delays other than autism spectrum disorder. Children were enrolled into the study from 2003 to 2006.
The children were part of the Northern California-based Childhood Autism Risks from Genetics and the Environment (CHARGE) study, for which Hertz-Picciotto is the principal investigator. Children aged 24-60 months are enrolled in the study, in which researchers are looking at a variety of exposures and their possible association with the disorder. Some of the participants have autism spectrum disorders, some have other developmental disorders, and a third group of children is typically developing, serving as study controls.
Her team looked at a variety of sources of mercury in the environment, including consumption of fish, use of personal care products that contain mercury such as nasal sprays and earwax removal products, and vaccinations. They also looked at whether children had mercury-based dental amalgam fillings.
Mercury Levels: Study Findings
The autism group did not differ from the typically developing group in the level of mercury circulating in their blood after the researchers adjusted for the sources of mercury, Hertz-Picciotto says.
''There are no obvious differences in the circulating levels of mercury" among the three groups, she says.
"Unadjusted, those with autism had lower levels as it turns out," she says. That may be due to a lower consumption of fish among those with autism, she says, perhaps because of the tendency to be picky eaters and adhere to the same foods.
The average levels of mercury were 0.24 micrograms per liter for the typically developing children, 0.26 micrograms per liter for those with autism or autism spectrum disorder, and 0.16 micrograms for those with other developmental disorders, she found.
To put that in perspective, Hertz-Picciotto says the Environmental Protection Agency considers a level of 5.8 micrograms per liter not risky for pregnant women. There is no specific standard set for children, she says.
Mercury Levels and Autism: Other Opinions
The new study drew mixed reactions from experts. The new findings ''should be reassuring to parents," says Geraldine Dawson, PhD, chief science officer for Autism Speaks and a research professor at the University of North Carolina, Chapel Hill.
''There has been concern by some parents that just the normal kind of exposure to mercury in our environment ... might have a bigger effect on children with autism, and they might have specific trouble metabolizing mercury,” Dawson says.
''This [study] suggests that children with autism are not retaining high levels of mercury in their body,” she says. “This study does not address the issue of whether mercury played a role in causing autism."
Sallie Bernard, co-founder and executive director of the Coalition for SafeMinds, called the study interesting but limited in its worth. "I think this is a study that adds to the literature on mercury and autism," she says. "But because it is looking at post-diagnosis exposure and does not investigate unique susceptibility and different toxicokinetics [such as absorption] of children with an autism spectrum disorder, the value of the study in understanding the role of mercury in autism is limited."
In a statement released by SafeMinds in response to the study, Bernard calls for research to look at how children with autism may handle mercury exposures differently, pointing to research suggesting children with autism may be more susceptible to stressors such as mercury.